Inside an internal combustion engine, a blowby gas occurs, which blows into a crankcase from a gap between a cylinder and a piston. A blowby gas contains an unburned HC component in a high concentration, and therefore, the blowby gas is not directly released into the atmosphere. In an ordinary internal combustion engine, a blowby gas is recirculated to an intake passage and is treated by re-combustion.
A blowby gas contains NOx generated by combustion. Therefore, depending on the concentration of NOx contained in the blowby gas, combustion of the internal combustion engine is likely to become worse when the blowby gas is recirculated to the intake passage. With regard to the problem, Japanese Patent Laid-Open No. 2006-138242 proposes to measure the NOx concentration of a blowby gas by a NOx sensor attached to a blowby gas recirculation passage, and stop the recirculation of the blowby gas to the intake passage when the NOx concentration exceeds an allowable limit.
Incidentally, a blowby gas has the characteristic of reducing the lubricating performance of an internal combustion engine by reacting with oil and a fuel. The main factor of the characteristic is NOx contained in a blowby gas. NOx causes polymerization reaction with oil and a fuel, and thereby, sludge is generated. The sludge generated in a crankcase degrades the lubricating characteristic of oil. Meanwhile, when the blowby gas is recirculated to an intake passage, sludge is generated in the intake passage by polymerization reaction of NOx and oil or a fuel. The sludge becomes a deposit and accumulates in the intake passage to worsen the intake efficiency of the internal combustion engine.
The generation amount of sludge correlates with the mass of NOx existing in a space around oil and a fuel. Accordingly, in performing suitable control by accurately diagnosing the state of the internal combustion engine, the mass of NOx can be said as important information. The mass of NOx in the crankcase can be represented by the NOx concentration in the crankcase. This is because the pressure and the volumetric capacity are constant in the crankcase, and there is no change in the mass of all the gases in the crankcase. Meanwhile, the mass (in detail, a mass flow rate) of NOx in the intake passage cannot be represented by the NOx concentration because in the intake passage, change of the pressure is large, and the mass flow rate of all the gases significantly changes. In order to diagnose the generation situation of the sludge in the intake passage, the mass flow rate itself of NOx which is recirculated to the intake passage with the blowby gas needs to be measured.
However, the method for accurately obtaining the mass flow rate of NOx in the intake passage has not been proposed so far. As described above, Japanese Patent No. 2006-138242 indicates that a sensor is disposed in the blowby gas recirculation passage to measure the NOx concentration, but mentions nothing about measurement of the mass flow rate of NOx. If the mass flow rate of NOx is obtained on the precondition of the art described in the publication, the mass flow rates of all blowby gases are needed as information. This is because the value obtained by multiplying the mass flow rates of all the blowby gases by the NOx concentration is the mass flow rate of NOx. However, the blowby gas recirculation passage is extremely slim as compared with the intake passage; and therefore, it is difficult to provide a mass flowmeter such as an air flowmeter. Further, there is a problem in attaching the NOx sensor to the blowby gas recirculation passage. Not only the circulation of the blowby gas is likely to be inhibited by the pressure loss increased by installment of the NOx sensor, but also measurement itself is unlikely to be accurately performed due to the influence of moisture.